1. Field of the Invention
The invention relates to an adsorption heat pump or an adsorption refrigeration machine and a heat accumulator suitable for efficient operation.
2. Background Description
The COP (Coefficient of Performance), i.e., the ratio of useful heat or useful cold to required driving heat, of adsorption refrigeration machines and adsorption heat pumps is typically lower than that of corresponding absorption plants under identical cycle conditions. A thermodynamic analysis (e.g., Meunier et al. 1996, 1997, 1998) shows that this lies in the adsorption above all at the entropy production, which occurs with a coupling of the adsorber to the heat sources and heat sinks of fixed temperature.
This is illustrated by the example of the refrigeration machine: the temperature of the adsorber fluctuates in the course of the cycle between the highest available desorption temperature and the lowest adsorption temperature at which the adsorption heat can still be emitted to the environment (recooling, e.g., in the cooling tower). The driving heat source (gas burner; solar system), however, generally supplies heat at a constant temperature level. The entropy production is thus greatest in the coldest state of the adsorber (at the start of the desorption phase). The situation with the entropy production during recooling is analogous: the recooler is generally kept at a constant temperature level, the entropy production is greatest here at the start of the adsorption phase, when the adsorber is hottest.
The entropy production for various adsorption cycles was examined in detail by the working groups around Meunier and Pons in France (see F. Meunier, F. Poyelle, M.D. LeVan: “Second Law Analysis of Adsorption Refrigeration Cycles: The Role of Thermal Coupling Entropy Production.” Applied Thermal Engineering 17, 43-55, 1997, and M. Pons, F. Poyelle: “Adsorptive machines with advanced cycles for heat pumping and cooling applications,” Internat. Journal of Refrigeration 22, 27-37, 1999). It was shown thereby that the entropy production can be clearly reduced and the COP clearly increased when the heat recovery is optimized. The aim is always to operate the adsorber in every operating condition with the lowest possible temperature difference to the heat source or heat sink. In practice, there is a minimal temperature difference that is required in order to be able to extract the desired output from the adsorber.
Essentially, two types are now proposed in the literature for realizing this heat recovery:
Firstly, a coupling of several adsorbers in a manner such that heat that is released in a still completely desorbed adsorber at the highest temperature level of the adsorption can be used for the desorption in another adsorber that is just at the start of its desorption phase.
Secondly, the realization of a “thermal wave” (U.S. Pat. No. 4,694,659) in a circuit with two adsorbers. A temperature gradient thereby passes through the two adsorbers switched one behind the other in a fluid circuit, wherein in the flow direction the reheater (the high temperature heat source) is switched between the adsorbing and the desorbing adsorbers and the recooler is switched between the desorbing and the adsorbing adsorbers (the average temperature heat sink). To switch over between desorption and adsorption, the flow direction of the fluid is reversed and the flow-through of recooler and reheater is switched over such that the aforementioned flow-through sequence results again.
The main disadvantage of the “thermal wave” is that the temperature gradient must be very steep for a significant increase of the COP, so that, e.g., during the largest possible part of the adsorption phase, the heat transfer fluid leaves the adsorber at the maximum adsorption temperature (and the amount of heat to be supplied by reheating is minimized). However, a steep temperature gradient also results in only a small part of the adsorber being active (i.e., adsorbs) at any time and the major part of the adsorber has either already completely adsorbed or has not yet started the adsorption. This has a negative effect on the power density of the refrigeration machine (specific cooling power, SCP). With the “thermal wave” there is therefore a marked conflict between a high COP and a high power density. Furthermore, with the “thermal wave” the entire adsorber must be flowed through serially, which results in long paths for the heat transfer fluid, and thus, high pressure losses and pump energy consumption. These problems may also have contributed to the fact that in the twenty years since the patent application of Shelton (U.S. Pat. No. 4,694,659) no adsorption heat pump or adsorption refrigeration machine has been brought on the market that realizes the principle of the “thermal wave.”
The main disadvantage of arrangements with heat recovery between more than two adsorbers is the high expenditure in terms of equipment that is to be operated for the adsorbers (to be thermally insulated from one another) and their changing interconnections. The achievable COP increases here with the number of adsorbers, at the same time, however, the expenditure in terms of equipment rises, and thus, the cost of the refrigeration machine/heat pump rises.
It can be generally stated with regard to the prior art for adsorption refrigeration machines that with respect to the technology of compression refrigeration machines that dominates the market above all, the power density (SCP) of the adsorption devices must be clearly increased further in order to achieve competitiveness. However, at the same time for many potential fields of application, the COP of the adsorption machines must also be increased in order, e.g., in the generation of the driving heat through a fossil fuel to achieve primary energy advantages with respect to current-driven compression devices.
In recent years marked progress has been achieved in the direction of a higher power density. Thus, e.g., SorTech AG developed a method for coating heat exchangers through the consumptive crystallization of zeolites on aluminum. This is described in DE 102004052976 A1 “Method for producing a substrate coated with a zeolite layer.” Through the close thermal contact between zeolite and heat exchanger metal sheet and the small thickness of the zeolite layer, the adsorption output that can be extracted from the heat exchanger can be clearly increased compared to a packing or adhesion of zeolite pellets. Unfortunately, this improvement in the power density is first obtained at the cost of a reduction of the COP. Namely, due to the thin zeolite layer, the mass ratio of adsorbent to heat exchanger, and thus, via the adsorption cycle, the heat ratio of sorptive to sensible heat is lower than with comparable systems with zeolite packing. With an unfavorable sorptive/sensible heat ratio, such as can be expected for systems that render possible a high power density, increased efforts to increase the COP are therefore necessary. This applies both to the recovery of the sensible heat fed to the adsorber during the desorption, as well as the reduction of the entropy production through the coupling of the adsorber to the external heat sources and heat sinks.
In the unexamined German application DE 199 08 666 A1 entitled “Adsorption heat pump/refrigeration machine with heating of the previous adsorber to desorption temperature by adsorption,” the use of a temperature-layered accumulator (stratified accumulator) in connection with an adsorption heat pump is described. It thereby relates primarily to a heat recovery between two evaporator/condenser components by a stratified accumulator. In the adsorption heat pump described here, the two adsorbers are operated according to the “thermal wave” principle. A component is permanently assigned to each adsorber, which component alternately takes over the function of the evaporator and condenser. Due to this special feature in the construction of the heat pump, through which valves between the adsorbers and evaporator/condenser can be omitted, the component carries out a corresponding temperature change with each change between evaporator and condenser function. Since the two components assigned to the two adsorbers change their function simultaneously, there is the possibility of a heat recovery between these two components. Different possibilities are described for realizing this heat recovery efficiently by a stratified accumulator.
Stratified accumulators are known from the prior art. Examples are described in DE 3905874 C2 and DE 10212688 A1 (Solvis) and EP 1076219 B1 (Sailer). According to their purpose of the stratification of heat from solar collectors and provision of heat for heating and domestic water, however, these accumulators do not contain any devices for temperature-controlled removal of fluid from selectable accumulator height.